Even the hippest chemist doesn’t know how many potentially world-changing chemistry start-ups are out there. As we at C&EN present our fifth class of 10 Start-Ups to Watch (two companies are founded by UC Berkeley faculty and alum), we can confirm that there are definitely hundreds, and perhaps thousands. That makes the job of picking just 10 a challenge—though an inspiring one. This year’s choices were selected after vigorous debate by our writers and editors. We made our own lists based on our day-to-day reporting and scoured the hundreds of firms nominated by readers and advisers from around the world. We picked winners for their groundbreaking chemistry as well as the importance of the problems they are tackling.